The present invention relates to a signal processing unit in a television system, and more particularly, to a signal processing unit for outputting one video signal out of two input video signals.
In recent years, such a multiplex television broadcast system has been developed that along with video signals corresponding to ordinary television images, multiplex broadcast signals such as video signals or coded signals corresponding to other images such as characters, figure patterns, etc. are also transmitted during vertical retrace periods, and on a receiver side, either of the ordinary television video signals and the multiplex broadcast signals are selected on demand to display either the ordinary television images or the other images transmitted as the multiplex broadcast. The multiplex broadcast images may be displayed on a part or whole of the image screen. In the United Kingdom and France, such systems have been put in practical use as "TELETEXT" and "ANTIOPE", respectively. In the United States and Japan, it is still in the experimental stage.
The multiplex broadcast signals corresponding to the multiplex broadcast images, which are transmitted during vertical retrace periods, have little information content because the vertical retrace period is short and signals necessitated for interlacing in the next field such as equalizing pulses, horizontal synchronizing pulses, etc. are also transmitted during the same period. Therefore, on the receiver side, the multiplex broadcast signals transmitted and received during the vertical retrace periods are first stored in a memory device. Then, video signals corresponding to the multiplex broadcast images are processed on the basis of the stored signals to obtain three primary color signals (R', G', B'). On the other hand, video signals corresponding to the ordinary television images are derived as three primary color signals (R, G, B) by means of a conventonal video signal processing circuit. Driving a picture tube by means of the three primary color signals, images are reproduced on the image screen of the picture tube. In the following, for the purpose of discriminating the primary color signals corresponding to the ordinary television images from the primary color signals corresponding to the multiplex broadcast images, the former are called simply "original primary color signals," while the latter are called "external primary color signals".
In order to display the multiplex broadcast images on a part or whole of the image screen of the picture tube in place of the ordinary television images, a signal switching unit is necessary for switching the original primary color signals and the external primary color signals to apply one of them to a picture tube. What is required for the signal switching unit is to maintain the pedestal potential and the maximum amplitude level between the original primary color signals and the external primary color signals at the same constant values so that the brightness and contrast of the images may be identical even when switching is effected between the ordinary television images and the multiplex broadcast images. Moreover, it is also important that the signal switching unit should be easily coupled to a video signal processing circuit for processing the video signals corresponding to the ordinary television images to derive the original primary color signals. The signal switching unit should preferably be formed as a monolithic integrated circuit with a few external parts. Furthermore, the signal switching unit is required not to be affected by the voltage variation in a reference voltage for giving a pedestal potential due to the change in the environmental temperature.
In some cases, the maximum amplitude level of the original primary color signals may be higher than that of the external primary color signals. For instance, in the "ANTIOPE" system in France, the maximum amplitude level of the original primary color signals is preset at 3 Vp-p (peak-to-peak), while that of the external primary color signals is preset at 1 Vp-p. In this case, if the maximum amplitude levels of the respective primary color signals are not equalized, a difference is observed in the displaced pictures, especially in their contrasts. In order to equalize the maximum amplitude levels of the respective primary color signals, by way of example, the external primary color signals (R', B', G') may be preliminarily amplified so as to be equalized to the amplitudes of the original primary color signals. However, this method has shortcomings in that the dynamic range of the signal switching unit becomes narrow and it is difficult to provide a contrast control circuit or the like subsequent to the signal switching unit. In another example, it is also possible to equalize amplitude level of the original primary color signals to that of the external primary color signals by reducing the gain of the video signal processing circuit for deriving the primary color signals from the ordinary color television image signals, and to then select the primary color signals by the signal switching unit and amplify them. However, according to this method, the gain of the video signal processing circuit is reduced, and therefore a leakage current of a carrier component contained in a demodulated color difference signal would become large. As a result, the S/N ratio of the displayed picture is degraded.
Thus, the signal switching unit for switching the primary color signals of the ordinary television images and the external primary colors signals of the multiplex broadcast images to output one primary color signal group must fulfil the above-mentioned essential conditions. However, a signal switching unit which can fulfil these essential conditions has not been provided in the prior art.